Generally, copying machines and printers employing an image-forming apparatus and forming an image in the electrophotographic process have been available commercially. An image is formed in such an image-forming apparatus in the following manner. Specifically, an image is formed by generating an electrostatic latent image on a photosensitive drum charged by photoirradiation, forming a toner image by adhesion of a toner on the electrostatic latent image, and transferring the toner image onto a recording paper. There are two methods of transferring the toner image; a method of transferring it directly from photosensitive member onto recording paper and a method of transferring once on intermediate transfer belt and then retransferring the image onto recording paper.
In a color copying machine or printer in the electrophotographic process, toner images in cyan, magenta, yellow, and black are formed respectively, giving a color image in combination thereof. Recently, widely used is a process of forming a color toner image once on an intermediate transfer belt from these toner images and retransferring the image onto recording paper (see, for example, Patent Document 1).
FIG. 6 is a sectional view illustrating the configuration of the image-forming apparatus disclosed in Patent Document 1. The image-forming apparatus is a so-called tandem-type image-forming apparatus, wherein: 10 represents an image-supporting photosensitive member; 11, a Scorotron charger; 12, an image-writing device; 13, a developing device; 14, a cleaning unit for cleaning the surface of the photosensitive member 10; 15, a cleaning blade; 16, a development sleeve; and 20, an intermediate transfer belt.
Each image-forming unit 1 has a photosensitive member 10, a Scorotron charger 11, a developing device 13, a cleaning unit 14, and others; the mechanical configurations of the image-forming units 1 in various colors are the same as each other; and thus in the Figure, reference numerals are allocated only to the image-forming unit in Y (yellow), and those for the elements in the image-forming apparatuses in M (magenta), C (cyan) and K (black) are eliminated.
The image-forming units 1 in various colors are placed in the order of Y, M, C, and K in the running direction of the intermediate transfer belt 20, and each photosensitive member 10 is in contact with the surface of the stretched intermediate transfer belt 20 and revolves at a constant linear velocity in the same direction as the running direction of the intermediate transfer belt 20 at the contact point.
The intermediate transfer belt 20 is stretched by a driven roller 21, a transfer roller 22, a tension roller 23, a charge-eliminating roller 27, a driving roller 24 and others, and the these rollers, the intermediate transfer belt 20, a transfer device 25, a cleaning unit 28, a transfer auxiliary roller 29 and others constitute a belt unit 3. The intermediate transfer belt 20 is driven by rotation of the driving roller 24, which in turn is driven by a drive motor not shown in Figure.
The photosensitive member 10 is a cylindrical metal base material, for example of aluminum, carrying an electrically conductive layer and a-Si layer or a photosensitive layer of an organic photoreceptor (organic photoconductor) formed on the external surface thereof and revolves counterclockwise in the direction indicated by the arrow in the Figure while the electrically conductive layer is grounded.
The electrical signal corresponding to the image data from the image-reading device 80 is converted into an optical signal by image-forming laser, and laser beam is irradiated on the photosensitive member 10 by the image-writing device 12.
The developing device 13 has a cylindrical nonmagnetic development sleeve 16 of stainless steel or aluminum revolving in the same direction as that of the photosensitive member 10, as separated from the peripheral surface of the photosensitive member 10 by a particular distance at the closest position.
The transfer device 25 has a function to transfer the toner image formed on the photosensitive member 10 onto the intermediate transfer belt 20, while a direct current at the polarity opposite to that of the toner is applied to the transfer device 25. A transfer roller may be used as the transfer device 25, instead of the corona discharger.
The numeral 26 represents a ground roller separable via the intermediate transfer belt 20 from the transfer roller 22 that retransfers the toner image formed on the intermediate transfer belt 20 onto an image-receiving medium P.
The cleaning unit 28 is installed at a position facing a driving roller 24 via the intermediate transfer belt 20. After transfer of the toner image onto the image-receiving medium P, the toner remaining on the intermediate transfer belt 20 is weakened electrostatically and separated from the intermediate transfer belt by a charge-eliminating roller 27 to which an AC voltage convoluted with a DC voltage at the polarity the same as or different from that of the toner is applied, and the separated toner is fed by a conveyer roller 282 onto a conveyer screw 283 and then discharged through a conveyer pipe into a discard duct.
The numeral 4 represents a fixing device, wherein: 41 represents a heating roller; 42, a pressure roller; 43, a guide plate; 44, a fixing-device paper discharge roller; and 81, a main-body paper discharge roller. The numeral 70 represents a paper delivery roller; 71, a timing roller; 72, a paper cassette; and 73, a conveyer roller.
In an image-forming apparatus in the configuration above, a color image is formed in the following image-forming process: First, simultaneously with initiation of image recording, the photosensitive member 10 of color signal Y revolves counterclockwise as indicated by the arrow by operation of the photosensitive member-driving motor not shown in Figure, and simultaneously, an electric potential is applied to the photosensitive member 10 by the electrostatic charging action of the Scorotron charger 11.
After application of the electric potential to the photosensitive member 10, an image corresponding to the Y image data is written by the image-writing device 12, leaving an electrostatic latent image corresponding to the Y document image on the surface of the photosensitive member 10.
The electrostatic latent image is reversely developed in the non-contact state by the Y developing device 13, forming a Y toner image on the photosensitive member 10 by rotation of the photosensitive member 10.
The Y toner image formed on the photosensitive member 10 is transferred onto the intermediate transfer belt 20 by action of the Y transfer device 25.
After the Y toner image is transferred on the intermediate transfer belt 20, the photosensitive member 10 is cleaned by the cleaning unit 14, before entering into the next image-forming cycle (subsequent M, C, or K cleaning process is the same as that above, and duplicated description is omitted).
Then, an image corresponding to the M (magenta) color signal, i.e., M image data is written by the image-writing device 12, giving an electrostatic latent image corresponding the M document image formed on the surface of the photosensitive member 10. The electrostatic latent image is converted to a M toner image formed on the photosensitive member 10 by the M developing device 13, which is then superimposed on the Y toner image by the M transfer device 25 synchronously with the Y toner image on the intermediate transfer belt 20.
In a similar manner, a C (cyan) toner image is transferred on the Y and M superimposed toner image synchronously with the Y and M superimposed toner image by the C transfer device 25, and a K (black) toner image is transferred on the Y, M and C superimposed toner image synchronously with the Y, M, and C superimposed toner image by the K transfer device 25, forming a Y, M, C and K superimposed toner image on the intermediate transfer belt 20. The intermediate transfer belt 20 carrying the superimposed toner image is sent clockwise as indicated by the arrow.
On the other hand, the image-receiving medium P is fed from a paper cassette 72 by a paper delivery roller 70, sent via a conveyer roller 73 to a timing roller 71, and fed by operation of the timing roller 71 to the transfer region S of a transfer roller 222, to which a DC voltage in the same polarity as that of the toner is applied (in contact with the ground roller 26), synchronously with the superimposed toner image on the intermediate transfer belt 20, transferring the superimposed toner image formed on the intermediate transfer belt 20 onto the image-receiving medium P. Then, the intermediate transfer belt 20 is processed with the charge-eliminating roller 27 for reduction of the charge on the residual toner and cleaned with a cleaning blade 281 in contact with the intermediate transfer belt 20, before entering into the next image-forming cycle.
The image-receiving medium P carrying the transferred superimposed toner image is sent to the fixing device 40 and held under heat and pressure in the nip region T between the heating roller 41 and the pressure roller 42, and then, the image-receiving medium P carrying the fused superimposed toner image is discharged by the paper discharge roller 81 into a paper discharge tray 82.
The surface of the intermediate transfer belt should be resistant to friction, because the intermediate transfer belt plays a role to carry and convey the superimposed toner image and transfer it favorably onto the image-receiving medium P as described above, and is subjected to the mechanical stress by cleaning blade 281 after transfer.
Patent Document 1 discloses that the intermediate transfer belt 20 is preferably an endless belt having a volume resistivity of 106 to 1012 Ω·cm and having a two-layer structure seamless belt of a base material and an anti-toner-filming layer. The base materials exemplified there include a semiconductive film having a thickness of 0.1 to 1.0 mm and containing an engineering plastic resin, such as modified polyimide, heat-hardening polyimide, ethylene tetrafluoroethylene copolymer, polyvinylidene fluoride or nylon alloy, and a conductive material dispersed therein, and a semiconductive rubber belt having a thickness of 0.5 to 2.0 mm and containing a silicone rubber, polyurethane rubber or the like and a conductive material dispersed therein; and the anti-toner-filming layer exemplified above is a fluorine-based coating film having a thickness of 5 to 50 μm. The conductive material commonly added to such an intermediate transfer belt is carbon black.
[Patent Document 1] Japanese Unexamined Patent Publication No. 2005-37586